1. Field of the Invention
The present invention relates to the field of wood pulp and papermaking. More particularly, the present invention relates to the semichamical pulping technique for making unbleached structural paper such as fluting medium for corrugated board.
2. Description of the Prior Art
Wood suitable for the manufacture of paper pulp generally comprises 45 to 55% alpha cellulose and 14-25% hemicellulose fibers bound structurally together with a 15 to 35% quantity of lignin, a tacky, thermoplastic, complex organic compound. Since the continuous formation of a paper web on a moving fourdrinier wire requires that the cellulosic wood fibers be deposited thereon in a 0.05 to 3% aqueous slurry, it is necessary to segregate the individual fibers from the matrix structure of natural occurrence. The process by which such segregation is performed is characterized as the pulping process.
Depending on the desired characteristics of the resulting paper product, wood pulping may be performed by exclusively mechanical means, thermochemical means or by a combination of the two, e.g. semichemical means. Characteristics distinctive of these processes include the degree of lignin removal and yield (weight of oven dry pulp per given weight of oven dry wood). Since mechanical pulping essentially constitutes a shredding process, lignin removal is negligible and yields are in excess of 95%. Chemical pulping, on the other hand, as the objective of removing as much lignin as practicable thereby yielding only 40 to 60% pulp from a wood charge.
The present invention relates to semichemical pulping wherein about half of the soluble lignin is removed for a yield of 70 to 85%.
Semichemical pulps are particularly useful and economical for the production of unbleached, high strength structural paper such as fluting medium for corrugated sheet and containers. In such applications, the residual lignin is functionally useful, due to the thermoplastic properties thereof, as a structural adjunct to strength and rigidity of the product.
Definitively, semichemical pulping is a two-step process that requires a mild single stage chemical treatment of wood chips to weaken the lignin bond on the cellulose fibers followed by a mechanical refining operation to complete the separation.
Historically, semichemical pulps were first made in the 1880's by chemically treating the wood chips with sulphurous acid or bisulphite followed by grinding.
Currently, such pulp is dominately prepared by the neutral or acid sulphite semichemical process wherein the primary chemical agent of delignification is a 120 to 200 grams/liter solution of sodium sulphite and a small quantity (30 to 50 grams/liter) of alkaline compound such as sodium carbonate, bicarbonate or hydroxide. The function served by the alkaline compound is to neutralize the released acids which are highly corrosive to process equipment.
The usual sulphite semichemical "cook" is conducted in batch digesters at 160.degree. to 190.degree. C for 1 to 3 hours with solution (liquor) to wood ratios in the order of 3-4:1 (4 gm liquor: 1 gm oven dry wood). This period may be shortened considerably, however, by a preimpregnation period followed by a 15 to 20 minute transit through a continuous digester at 200.degree. C. Such preimpregnation treatments will be subsequently discussed in greater detail.
From the limited perspective of mere pulp preparation, the sulphite semichemical process is very economical in comparison to the product value. Only the barest minimum of chemical and mechanical energy is expended with relatively simple capital equipment. From the broader view of waste chemical recovery and the consequent ecological effect of sewering such wastes, the otherwise economically attractive sulphite process becomes more marginal. Chemical pulping processes, specifically the "kraft" or alkaline sodium sulphite-sodium hydroxide process, have, by virtue of greater lignin removal, sufficient heat value in combustible organics to support an incineration process of cooking chemical recovery. To recover used cooking chemicals from the sulphite process in a similar manner requires the addition of purchased fuels.
In recent years, it has been discovered that by operating a sulphite pulping operation in conjunction with a kraft operation of 3 to 4 times greater production, expended chemical from both processes may be recovered from a common or "cross" recovery plant. This solution is not won without difficulty, however, due to the cyclic accumulation of sodium ion in the kraft system. Eventually, the system must be purged which means the sewering of a considerable quantity of expensive chemicals and the attendant ecological shock.
Next in the evolution of semichemical pulping came the technique of directly sulfiting green liquor from a kraft-semichemical cross-recovery plant. As in the usual kraft recovery process, black liquor, the collodial, fluidized substance drained from a finished "cook" of wood pulp, is evaporatively concentrated to approximately 60% solids and the combustible organics thereof ignited in an incinerator to generate heat for the foregoing evaporation step. The residue of combustion predominately comprises sodium carbonate and sodium sulfide. When solublized, this residue, or smelt, is characterized as green liquor.
To continue the kraft recovery, the green liquor is subsequently causticized with lime and clarified to convert most of the sodium carbonate to sodium hydroxide.
In the direct sulfitation of green liquor to compound semichemical cooking liquor, a portion of the solublized smelt following incineration is contacted with sulfur dioxide or other reactive sulfur compound to dissociate some of the sodium from the carbonate and generate sodium sulfite: a compound generally considered essential to the semichemical pulping process.
Although the direct sulfitation of green liquor solves the problem of accumulated sodium ion in an otherwise closed system, it substitutes in lieu thereof a sulfur accumulation problem. Moreover, during the direct sulfitation step of the process, a complex interaction between the existing sodium sulfide and dissociated carbonate occurs to generate sodium thiosulfate and hydrogen sulfide. Thiosulfate is an inhibiting agent to the kraft cooking process and hydrogen sulfide, which escapes to the atmosphere, is an odoriferous, toxic, air pollutant.
In view of such prior art history, what has been desired by the semichemical wood pulping industry is a truly closed recovery system for cooking chemicals and preferably one that may be operated, without regard to relative production rates, in conjunction with a kraft system as a cross-recovery plant. As a further desideratum, in view of recently imposed air and stream polluting mill effluent regulations, is a semichemical pulping process that requires little sulfur as an active pulping chemical. An ideal semichemical pulping process would comprise both elements, e.g. a low sulfur cook adaptable to a closed cycle chemical recovery.
The most obvious and direct approach to the problem by a semichemical pulp mill having kraft process recovery facilities available is with the kraft system green liquor. However, a green liquor per se has been considered unsuitable as lacking sufficient active sulfur for wood delignification. For this reason, Richter et al, in their U.S. Pat. No. 2,694,631 contended that "Up to now, no method of manufacturing chemical wood pulp is known in which the green liquor can be used as a cooking liquor for the wood, this being the sole cooking liquor." Unfortunately, Richter et al did not unequivocally succeed, either, since their green liquor cooking process specified a 4 to 10 hours, 100.degree.-110.degree. C, 150-200 psi immersed pretreatment of the chips in an aqueous solution of 12-30% sulfur dioxide. The Richter et al. product objective, however, was a 33% yield alpha cellulose pulp, a product having substantially different properties and uses from those of 70% or greater yield semichemical pulp. Since semichemical cooking times are less than 3 hours in batch digesters, and in continuous digesters, less than 60 minutes and often as short as 5 minutes, the Richter et al disclosure is of little significance to the semichemical industry. Moreover, the sulfur dioxide pretreatment is prohibitive, ecologically if not economically.
Another investigator, Steinar Vardheim, disclosed in the 1967, Vol. 9 issue of Paper ach Tra, a more practical semichemical green liquor cooking process whereby birch chips were cooked, following a 13 minute, 80.degree. C preimpregnation period, in the vapor phase at 150.degree. C for 5 minutes with a 66 to 150 grams/liter sodium ion concentration (expressed as Na.sub.2 O) of green liquor having 53-74% sodium carbonate, 24-50% sodium sulfide, and 2-14% sodium hydroxide. Vardheim's green liquor was combined with the chips at such a rate as to effect 7.9 grams of sodium ion (as Na.sub.2 O) per 100 grams of O.D. wood.
Although the mechanical properties of paper formed by Vardheim's pulp are adequate for the purpose intended, the consequent mill and paper odor is intolerable. Since the odor source is hydrogen sulfide and resultant methyl mercaptans, such emissions in large and continuous quantities are dangerously toxic to humans.